The present invention relates to a method and apparatus for protecting an electrical system against electrical shock in the event of a ground fault, and also for continuously monitoring the electrical system to assure it is operating in a proper and safe manner.
Electrical shock results from closing a current carrying loop through the body. Usually, the loop consists of touching an electrical device which has a ground fault therein causing electrical current to flow through the body to ground. The electrical current flowing through the body causes physiological effects which, depending on the intensity and time duration of the current, can be extremely hazardous and even fatal. For example, currents below 0.5 mA, commonly called the "perception threshold", usually produce no sensation or damage; currents from about 0.5 mA to about 16 mA, commonly called the "let go threshold" generally produce a relatively weak electrical shock sufficient to produce sensation but insufficient to block the muscles so that the person receiving the shock may still "let go"; currents from 16 mA to about 50 mA may result in extreme pain, possible fainting, exhaustion injury, and paralysis of the muscles preventing the person from letting go; whereas currents of above 50 mA may expose the person to ventricular fibrillation and death.
When a person first contacts a "hot" device, the current at first is very low because of the contact impedance which is very high at the instant of "touch" due to the low pressure applied, the small contact area, and the high skin resistance. The current at this time may be below the perception threshold. However, as the pressure of contact with the "hot" device is increased, the contact area increases and the skin resistance decreases (the skin resistance being inversely proportional to the current), thereby increasing the current exponentially. The electrical shock sensation produced may cause the person to release himself from the device if he reacts fast enough before the current has reached the "let go" threshold. If not, his reaction may be blocked, preventing him from "letting go".
One method of providing an electrical system with protection against electrical shock in the event of a ground fault is to include a ground fault detector and interrupter in the central mains inlet which detects leakage current to ground and interrupts the electrical system supplied from the central mains inlet. However, since such interrupters are applied to the central mains inlet and therefore serve a large number of electrical devices, their sensitivity cannot be any better than the inherent leakage of the overall system including all the local electrical devices connected to the system. In addition, the starting currents for any of the electrical devices may be very high for short intervals, such that any slight imbalance in the system could actuate the interrupter. Also, transients during the starting of the device introduce high frequency components which also tend to actuate the interrupter. As a result, the presently used ground fault detectors and interrupters are usually characterized by relatively poor sensitivity and reliability and/or a high rate of false interruptions.
An object of the present invention is to provide apparatuses having advantages in the above respects for providing an electrical system with protection against electrical shock in the event of a ground fault, and also for continuously monitoring the electrical system to assure it is operating in a proper and safe manner.